Eyepiece optical system, and display device using the eyepiece optical system

ABSTRACT

The invention relates to a display system wherein a Fresnel lens that forms an eyepiece optical system is curved to correct for decentration aberration, whereby a display screen can be wholly observed even in a decentered state. In the display system, an object image is projected via a relay optical system  2  near the eyepiece optical system  1  so that the exit pupil E 1  of the relay optical system  2  is projected via the eyepiece optical system  1  onto an observer&#39;s pupil position E 0 . The eyepiece optical system comprises an optical element having a Fresnel surface. The eyepiece optical system  1  and relay optical system  2  are located such that an axial chief ray emerging from the relay optical system  2  is obliquely incident on the eyepiece optical system  1 . The axial chief ray is defined by a light ray emerging from the center of the object, and passing through the relay optical system  2  and then through the center of the pupil E 1  of the relay optical system  2 . The optical element that forms the eyepiece optical system  1  is curved such that decentration aberration of pupil aberrations occurring at the eyepiece optical system  1  is corrected.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to a display system, andmore particularly to a display system of compact size and low powerconsumption.

[0002] Among compact display systems, there is a direct-viewing typeliquid crystal display system. These compact display systems, for themost part, are used with cellular phones and portable terminals. Forhigh-definition display purposes, on the one hand, display systemscomprising an increased number of pixels are needed. For moving imagedisplay purposes, on the other hand, display systems having fast displayspeeds are required. Such requirements are satisfied by use of activematrix liquid crystals. However, the active matrix liquid crystals areexpensive, and consume large power with the need of large capacitybatteries for presenting displays over an extended period of time.

[0003] Some arrangements using a small display device and designed topresent images appearing on that display device on an enlarged scalethrough an optical system are disclosed in JP-A 48-102527, and JP-A5-303054 filed by the applicant. In these arrangements, the imagesappearing on the display systems are magnified through a concave mirrorand displayed as virtual images. In the latter arrangement inparticular, a non-rotationally symmetric reflecting surface is used toobtain projected images with reduced aberrations.

[0004] There is also available a projection optical system proposed bythe applicant in JP-A's 5-303055 and 2000-221440. In this projectionoptical system, an image displayed on a display device is once projectedin midair to form a projected image. Then, the projected image ismagnified by a concave mirror for display purposes.

[0005] Display systems, for instance, are disclosed in JP-A's 7-270781and 9-139901.

[0006] Further, the applicant has already filed Japanese PatentApplication No. 2001-66669 to come up with a compact, low powerconsumption display system. In this display system, a relay opticalsystem and an eyepiece optical system are used to set up an opticalsystem. In this optical system, the relay optical system comprises adecentering prism optical system. Then, an image or its intermediateimage (hereinafter called simply the image appearing on the displaydevice is projected near the eyepiece optical system. The eyepieceoptical system also serves to converge a light beam from the relayoptical system toward the eyeball of an observer. At this time, theeyepiece optical system projects the exit pupil of the relay opticalsystem onto a given position. Here the given position is understood tomean the position of the eyeball of the observer upon observation.

[0007] For the optical system comprising a relay optical system and aneyepiece optical system, the eyepiece optical system must be decenteredso as to reduce its overall size. Then, the relay optical system islocated such that light rays emerging therefrom are obliquely incidenton the eyepiece optical system. The relay optical system is alsopositioned such that its exit pupil is located at either one of twofocuses F, F′ of such a spheroid as shown in FIG. 1. In this state, theeyeball of the observer is brought in alignment with the position ofanother focus (F or F′). Even in the decentered arrangement, there isthus no pupil aberration at all.

[0008] However, the eyepiece optical system must be constructed of alarge concave mirror that has a large thickness and so offerstroublesome problems in connection with portability and handleability.

[0009] To avoid these problems, it is known to construct the eyepieceoptical system using a transmission or reflection type Fresnel lens.

SUMMARY OF THE INVENTION

[0010] The present invention provides an eyepiece optical systemcomprising a substrate with a Fresnel surface formed thereon, wherein:

[0011] the Fresnel surface comprises rotationally symmetric concentriczones, and

[0012] the substrate includes at least a curved area.

[0013] The present invention provides an eyepiece optical systemcomprising a substrate with a Fresnel surface formed thereon, whereinthe Fresnel surface comprises a rotationally symmetric concentric zone,and the substrate is configured in a plane-parallel shape, and

[0014] a holder member for holding the substrate in place, wherein theholder member has a recess in which the substrate is held.

[0015] Further, the present invention provides a display systemcomprising:

[0016] a display device comprising a display part on which an image isto be displayed,

[0017] a relay optical system for projection of the image, and

[0018] the aforesaid eyepiece optical system, wherein:

[0019] the relay optical system and the eyepiece optical system arelocated such that an axial chief ray emerging from the relay opticalsystem is obliquely incident on the eyepiece optical system.

[0020] It is here noted that the axial chief ray is defined by a lightray that emerges from the center of the display part, and passes throughthe relay optical system, passing through the center of an exit pupil ofthe relay optical system.

[0021] Still other objects and advantages of the invention will in partbe obvious and will in part be apparent from the specification.

[0022] The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts, which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is illustrative of two focuses of a spheroid.

[0024] FIGS. 2(a) and 2(b) are illustrative in schematic of the Fresnelsurface used in the present invention.

[0025]FIG. 3 is illustrative in schematic of a display systemconstructed by curving the Fresnel reflecting mirror according toExample 1 of the present invention.

[0026]FIG. 4 is illustrative in schematic of a display system in whichthe Fresnel reflecting mirror according to a comparative example of FIG.3 is not curved.

[0027]FIG. 5 is an optical path diagram for a Y-Z section of the opticalsystem that underlies Example 1.

[0028]FIG. 6 is a projection optical path as projected onto the X-Zplane of the optical system that underlies Example 1.

[0029]FIG. 7(a) and 7(b) are decentration aberration diagrams forExample 1.

[0030]FIG. 8 is a decentration aberration diagram for Example 1.

[0031]FIG. 9 is illustrative of another embodiment of how to curve theFresnel reflecting mirror according to Example 1.

[0032]FIG. 10 is an optical path diagram for a Y-Z section of theoptical system that underlies Example 2.

[0033]FIG. 11 is a projection optical path as projected onto the X-Zplane of the optical system that underlies Example 2.

[0034]FIG. 12(a) and 12(b) are decentration aberration diagrams forExample 2.

[0035]FIG. 13 is a decentration aberration diagram for Example 2.

[0036]FIG. 14 is a view similar to FIG. 1, showing Example 2 of thepresent invention.

[0037]FIG. 15 is illustrative of another embodiment of how to curve theFresnel reflecting mirror according to Example 2.

[0038]FIG. 16 is illustrative of how to curve the Fresnel reflectingmirror according to Example 3 of the present invention.

[0039]FIG. 17 is illustrative of how to curve the Fresnel reflectingmirror according to Example 4 of the present invention.

[0040]FIG. 18 is illustrative of how to curve the Fresnel reflectingmirror according to Example 5 of the present invention.

[0041] FIGS. 19(a) and 19(b) are illustrative of the mechanism formounting the Fresnel reflecting mirror according to Example 6 of thepresent invention at a predetermined position in a given attitude.

[0042]FIG. 20 is an optical path diagram for Example 7 of the presentinvention.

[0043]FIG. 21 is illustrative of one exemplary application of thedisplay system according to the present invention.

[0044]FIG. 22 is illustrative of another exemplary application of thedisplay system according to the present invention.

[0045]FIG. 23 is illustrative of a further exemplary application of thedisplay system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] First of all, the Fresnel surface used herein is explained. AFresnel surface is defined by a basic curved surface that is cut into anumber of slender ring-like faces, in which the slender ring-like facesare arranged in the form of zones. The Fresnel surface used herein isdefined by a basic curved surface of rotationally symmetric shape, asshown in FIGS. 2(a) and 2(b). FIG. 2(a) is a perspective view of aFresnel surface 60 used herein, and FIG. 2(b) is a longitudinallysectioned view of one section of the Fresnel surface 60, including itscenter.

[0047] In the embodiment of FIG. 2(a), a rotationally symmetric Fresnelsurface is achieved by making the Fresnel pitch conform to arotationally symmetric spherical shape. The Fresnel surface 60, ifconfigured in the form of a refracting surface, provides a Fresneltransmitting surface, and if configured in the form of a reflectingsurface, provides a Fresnel reflecting surface. The Fresnel reflectingsurface is also obtainable by using the Fresnel surface 60 as a Fresneltransmitting surface and locating another optical surface in proximityto the Fresnel transmitting surface as a reflecting surface.

[0048] A reflecting mirror having such a Fresnel reflecting surfaceprovides a Fresnel reflecting mirror. On the other hand, a lens havingthe Fresnel transmitting surface provides a Fresnel lens. As can beappreciated from the examples given later, such a Fresnel reflectingmirror or lens is used herein as an eyepiece optical system.

[0049] The eyepiece optical system of the present invention, and thedisplay system using the eyepiece optical system are now explained withreference to their examples. In Example 1 of the present invention, aFresnel reflecting surface whose Fresnel surface is defined by aspherical surface is used for the eyepiece optical system.

[0050]FIG. 3 is illustrative of Example 1 of the present invention. FIG.4 is illustrative of a comparative example for Example 1. FIGS. 5 and 6are illustrative in detail of the comparative example. FIG. 5 is anoptical path diagram for a Y-Z section of the optical system in thecomparative example. FIG. 6 is a projection optical path diagram asprojected onto the X-Z plane. In FIGS. 5 and 6, the defining coordinatesfor the second surface (the plane of a Fresnel reflecting mirror 1 onthe entrance side) are indicated by X, Y and Z. It is noted that X, Yand Z in FIGS. 5 and 6 stand for coordinate axes; that is, they are notdecentering parameters X, Y and Z.

[0051] In FIGS. 5 and 6, only essential members, i.e., an exit pupil E1of a relay optical system, a Fresnel reflecting mirror 1 and a finalpupil E0 are shown. The final pupil E0 is an image of the exit pupil E1of the relay optical system.

[0052] The Fresnel reflecting mirror 1 comprises a plane 61 on theentrance side and a Fresnel reflecting surface 62, so that by theFresnel reflecting mirror 1, the image of the exit pupil El of the relayoptical system is formed at a given position where the final pupil E0 isto be formed. That position is also in alignment with the eyeball(pupil) of an observer upon observation, as already described. Thus, theFresnel reflecting mirror 1 functions as an eyepiece optical system.

[0053] FIGS. 7(a) and 7(b) and FIG. 8 are decentration aberrationdiagrams for the comparative example.

[0054]FIG. 7(a) is illustrative of aberration when a light ray leavingthe exit pupil E1 of the relay optical system is diverted on the Fresnelreflecting mirror 1 in the Y direction (PY). This aberration isindicative of Y-direction aberration (EY) at the position of the finalpupil E0. In this case, aberration of about 20 mm at most occurs.

[0055]FIG. 7(b) is illustrative of aberration when a light ray leavingthe exit pupil E1 of the relay optical system is diverted on the Fresnelreflecting mirror 1 in the X direction (PX). This aberration isindicative of Y-direction aberration (EY) at the position of the finalpupil E0. In this case, aberration of about 5 mm at most occurs.

[0056]FIG. 8 is illustrative of aberration when a light ray leaving theexit pupil E1 of the relay optical system is diverted on the Fresnelreflecting mirror 1 in the X direction (PX). This aberration isindicative of X-direction aberration (EX) at the position of the finalpupil E0. In this case, aberration of about 5 mm at most occurs.

[0057] It is here noted that the effective diameter of the Fresnelsurface 62 is 300 mm in the horizontal (X) direction and 225 mm in thevertical (Y) direction.

[0058] In this example, the Fresnel reflecting mirror 1 is so curvedthat decentration aberration occurring at the Fresnel reflecting mirror1 is corrected (or compensated)

[0059] Referring to FIG. 3, the display system comprises a displaydevice 3 for displaying an image, a relay optical system 2 and a Fresnelreflecting mirror 1. The image appearing on the display device 3 isprojected by way of the relay optical system 2 so that a projected imageis formed near the Fresnel reflecting mirror 1. The Fresnel reflectingmirror 1 reflects light from the projected image formed near itself and,at the same time, projects an exit pupil E1 of the relay optical system2 onto the position of a final pupil E0. Thus, the Fresnel reflectingmirror 1 reflects the light from the projected image toward the finalpupil E0.

[0060] If an observer brings the eyeball in alignment with the positionof the final pupil E0, the observer will be capable of viewing the imageappearing on the display device 3. At this time, the relay opticalsystem 2 and the Fresnel reflecting mirror 1 take the form of amagnifying optical system. Thus, the observer will be capable of abright, magnified image.

[0061] A difference between the Fresnel reflecting mirror 1 of FIG. 3and the Fresnel reflecting mirror 1 of the FIG. 4 lies in the shape oftheir lower end portions. That is, in the Fresnel reflecting mirror 1 ofFIG. 3, the lower end portion (−Y portion) of the planar Fresnelreflecting mirror 1 of FIG. 4 is curved away from the final pupil E0.Referring here to FIG. 7(a), decentration aberration occurs in apositive direction at a negative position on abscissa (in the Ydirection of a display surface (the Fresnel reflecting mirror 1)). Byuse of the Fresnel reflecting mirror 1 of FIG. 3, it is thus possible tomake correction for decentration aberration occurring in the positivedirection.

[0062] The decentration aberration shown in FIG. 8 is an astigmaticdifference caused by decentration. In this state, light rays near theoptical axis of the Fresnel reflecting mirror 1 form an image fartheroff an image plane (farther off the Fresnel reflecting mirror 1 in thiscase). For correction of this, it is preferable to cylindrically curvethe Fresnel reflecting mirror 1. Such a configuration enables correctionof the aforesaid decentration aberration.

[0063] More preferably, only the central portion of the reflectingsurface of the Fresnel reflecting mirror 1 should be cylindricallyconfigured while the peripheral portion remains in a substantiallyplanar shape, as depicted in FIG. 9. Decentration aberration issusceptible to over-correction at the periphery of the Fresnelreflecting mirror 1; however, such a configuration can foreclose thepossibility of over-correction.

[0064] Example 2 of the present invention is now explained. Numericaldata that underlie this example will be given later. In this example, aFresnel reflecting mirror having a rotationally symmetric asphericsurface is used for an eyepiece optical system.

[0065]FIG. 10 is an optical path diagram for a Y-Z section of theoptical system that underlies Example 2, and FIG. 11 is a projectionoptical path diagram as projected onto an X-Z plane. Only essentialmembers, i.e., an exit pupil E1 of a relay optical system, a Fresnelreflecting mirror 1 and a final pupil E0 are shown in FIGS. 10 and 11.

[0066] The Fresnel reflecting mirror 1 comprises a plane 61 on theentrance side and a Fresnel reflecting surface 62, so that by theFresnel reflecting mirror 1, the image of the exit pupil E1 of the relayoptical system is formed at a given position where the final pupil E0 isto be formed. That position is also in alignment with the eyeball(pupil) of an observer upon observation, as already described. Thus, theFresnel reflecting mirror 1 functions as an eyepiece optical system.

[0067] Decentration aberration in the arrangement of FIG. 10 is depictedin the aberration diagrams, i.e., FIG. 12(a), FIG. 12(b) and FIG. 13that are similar to FIG. 7(a), FIG. 7(b) and FIG. 8, respectively.

[0068] The effective diameter of the Fresnel reflecting surface 62 is300 mm in the horizontal (X) direction and 225 mm in the vertical (Y)direction.

[0069] In this example, the Fresnel reflecting surface 62 is configuredin the form of a rotationally symmetric aspheric surface. This enablesthe curvature of the Fresnel surface in the Y direction to be relativelyfreely determined. It is consequently possible to reduce the amount ofdecentration aberration produced in this direction as much as possible.This will also be appreciated from the optical path diagram of FIG. 10showing that the ability of light rays to converge is satisfactory. FromFIG. 12(a) showing that the amount of decentration aberration is barelyabout 3 mm, it will be found that the amount of decentration aberrationis kept small. It is noted that FIG. 7 differs from FIG. 12 in terms ofthe value of a graduation on ordinate.

[0070] For correction of decentration aberration, the Fresnel reflectingmirror 1 should be configured as shown in FIG. 14. In FIG. 14, a Fresnelreflecting mirror 1 is curved at its lower end (−Y) portion toward afinal pupil E0.

[0071] In this example, too, aberration remains in the X direction asshown in FIG. 13. This is because an astigmatic difference due todecentration in the X direction is still not well corrected. The amountof decentration aberration produced is about 8 mm as shown in FIG. 13.

[0072] In that case, the Fresnel reflecting mirror 1 should preferablybe curved such that it takes a cylindrical form in the X direction. Sucha form makes correction of the aforesaid decentration aberrationfeasible.

[0073] In addition, it is preferable to satisfy the following condition(1).

0<|E/EPD|<2  (1)

[0074] Here EPD is the diameter of an exit pupil E1 of a relay opticalsystem 2, and E is the amount of aberration at the position of the finalpupil E0.

[0075] When the Fresnel reflecting mirror 1 is curved in such a way asto satisfy the aforesaid condition (1), it is possible to correct orcompensate for astigmatic differences and coma caused by decentration.It is consequently possible to achieve a display system that is soreduced in pupil aberration that the whole display surface can be wellobserved.

[0076] In the instant example, the angle of decentration is 25.5°. Withan increasing angle of decentration, the amount of decentrationaberration produced becomes drastically large. For instance, assume nowthat the diameter of the final pupil E0 is 10 mm. It is then preferableto curve the Fresnel reflecting mirror 1 or the Fresnel lens in such away that the amount of aberration is reduced down to 20 mm or less. Thisenables the amount of aberration produced to be reduced whether theangle of decentration becomes greater or smaller than 22.5°.

[0077] More preferably, the following condition (1-1) should besatisfied.

0<|E/EPD|<1  (1-1)

[0078] To enlarge the pupil of the relay optical system 2, it ispreferable to locate an optical surface having diffusion characteristicsnear the Fresnel surface. Here the diffusion characteristics arerepresented in terms of a given curve (graph) with diffusion angle asabscissa and light intensity as ordinate. If the diffusion angle of thatoptical surface is less than 10° (the full width half maximum angle),the optical surface can then have a relatively weak diffusioncapability. In this case, it is important to satisfy the aforesaidcondition (1-1) because there is noticeable pupil aberration.

[0079] The numerical data that provide the bases of the eyepiece opticalsystems according to Examples 1 and 2 are set out below.

[0080] In the following tables for constitutive parameters, “FS”, “ASS”,and “RE” indicate a Fresnel surface, an aspheric surface, and areflecting surface, respectively.

[0081] It is here noted that the aspheric surface is defined by arotationally symmetric aspheric surface given by the following definingformula (a):

Z=(y ² /R)/[1+{1−(1+K)y ² /R ²}^(1/2) ]+Ay ⁴ +By ⁶ +Cy ⁸ +Cy ¹⁰+  (a)

[0082] where Z indicates an optical axis (axial chief ray) provided thatthe direction of propagation of light is defined as positive, yindicates a direction vertical to the optical axis, R is a paraxialradius of curvature, K is a conical coefficient, and A, B, C and D arethe fourth-, sixth-, eighth- and tenth-order aspheric coefficients. TheZ-axis in that defining formula gives the axis of the rotationallysymmetric aspheric surface.

[0083] How to give decentration to the numerical data is now explained.At a position spaced away from a certain surface I by its thickness inthe Z-axis direction there is a basic coordinates for I+1 surface. At aposition decentered from the basic coordinates by the amount ofdecentration of the I+1 surface (decentering X, Y, Z and tilt α, β, γ),there is a defining coordinates for the I+1 surface. Then, the shape ofthe I+1 surface is determined by that defining coordinates.

[0084] Next, on the basis of the defining coordinates for the I+1surface, a basic coordinates for I+2 surface is taken at a positionspaced away by the surface thickness in the Z-axis direction. As is thecase with the I+1 surface, the I+2 surface is defined by a definingcoordinates defined by the amount of decentration. The same goes truefor the subsequent surfaces. In other words, decentration is given on anintegrative basis.

[0085] The decentering parameters X, Y, Z are the amounts ofdecentration in the X-, Y- and Z-axis directions at the basiccoordinates, and the tilt parameters α, β, γ (°) are the angles of tiltaround the X-, Y- and Z-axes. In that case, the positive direction for αand β is given by counterclockwise rotation with respect to the positivedirection of the respective axes, and the positive direction for γ isgiven by clockwise rotation with respect to the positive direction ofthe Z-axis. It is noted that the parameters α, β and γ are rotated inthe order of counterclockwise a rotation of the basic coordinates aroundthe X-axis, then counterclockwise β rotation of a new coordinates aroundthe Y-axis, and finally clockwise γ rotation of new another coordinatesaround the Z-axis.

EXAMPLE 1

[0086] Surface Radius of Surface Displacement Refractive Abbe's No.curvature separation and tilt index No. 1 ∞ (Object) 600.00 2 ∞ 1.00 (1)1.4924 57.6 3 −799.23 −1.00 (2) 1.4924 57.6 (FS, RE) (Stop) 4 ∞ −450.00(3) 5 ∞ (Image) Displacement and tilt(1) X 0.00 Y 0.00 Z 0.00 α 22.50 β0.00 γ 0.00 Displacement and tilt(2) X 0.00 Y 103.06 Z 0.00 α 0.00 β0.00 γ 0.00 Displacement and tilt(3) X 0.00 Y −103.06 Z 0.00 α 0.00 β0.00 γ 0.00

EXAMPLE 2

[0087] Surface Radius of Surface Displacement Refractive Abbe's No.curvature separation and tilt index No. 1 ∞ (Object) 600.00 2 ∞ 1.00

(1) 1.4924 57.6 3 −798.59 −1.00

(2) 1.4924 57.6 (FS, RE) (Aspheric) (Stop) 4 ∞ −450.00

(3) 5 ∞ (Image) F S Aspherical Coefficients K = −0.59553 A = −3.93600 ×10⁻¹⁰ B = 1.16704 × 10⁻¹⁴ C = −4.58343 × 10⁻²⁰ Displacement and tilt(1)X 0.00 Y 0.00 Z 0.00 α 22.50 β 0.00 γ 0.00 Displacement and tilt(2) X0.00 Y 102.86 Z 0.00 α 0.00 β 0.00 γ 0.00 Displacement and tilt(3) X0.00 Y −102.86 Z 0.00 α 0.00 β 0.00 γ 0.00

[0088] Example 3 of the present invention is now explained. When aFresnel reflecting mirror is used as an eyepiece optical system, it ispreferable to make the Fresnel reflecting mirror so thin that theFresnel surface can more easily be curved. The instant example isdirected to curving the Fresnel reflecting mirror.

[0089] For correction of the astigmatic difference caused bydecentration, it is effective to cylindrically curve the substrate ofthe Fresnel reflecting mirror 1, as already explained with reference toExamples 1 and 2. In the instant example, a keeper frame 11 is used asshown in FIG. 16.

[0090] The keeper frame 11 is provided at both ends with raised edgeswith a recess formed between them. The recess has a flat bottom surface.The length of the recess between the raised edges is so slightly shorterthan the length of one side of the Fresnel reflecting mirror 1 that uponthe Fresnel reflecting mirror 1 fitted into the recess, given lateralforce is applied from both sides of the Fresnel reflecting mirror 1 tothe recess. This in turn enables the Fresnel reflecting mirror 1 to becurved in a given form. At the same time, the keeper frame 11 functionsas a holder for holding the Fresnel reflecting mirror 1 in place.

[0091] In the instant example, the force for holding the Fresnelreflecting mirror 1 in place can be controlled by an appropriate choiceof the length of the recess. In other words, the amount of curvature ofthe Fresnel reflecting mirror 1 can properly be determined. According tothe instant example, it is thus possible to optimize the amount of theaberration to be corrected in compliance with the amount of decentrationof the Fresnel reflecting mirror 1 and, consequently, to correct fordecentration aberration over a wider correction range.

[0092] It is then preferable to satisfy the following condition (2).

t/ED<0.05  (2)

[0093] Here ED is the diagonal length of the Fresnel reflecting mirror 1and t is the thickness of the Fresnel reflecting mirror 1.

[0094] As the upper limit of 0.05 to the aforesaid condition (2) isexceeded, the substrate of the Fresnel reflecting mirror 1 becomesthick, resulting in difficulty being encountered in curving the Fresnelreflecting mirror 1 in the given form.

[0095] More preferably, the following condition (2-1) should besatisfied.

t/ED<0.01  (2-1)

[0096] If this condition is satisfied, it is easier to curve the Fresnelreflecting mirror 1.

[0097] Example 4 of the present invention is now explained. In thisexample, too, the Fresnel reflecting mirror is curved. In the instantexample, a Fresnel reflecting mirror holder frame 12 is used. TheFresnel reflecting mirror holder frame 12 is similar in structure to thekeeper frame 11 in Example 3. For correction of aberration caused bydecentration, it is required to curve a Fresnel reflecting mirror 1 asalready explained. The amount of curvature of the Fresnel reflectingmirror 1 can be pre-calculated by means of simulation or the like.

[0098] In the instant example, the bottom surface of the Fresnelreflecting mirror holder frame 12 is curved on the basis of thepre-calculated amount of curvature, as shown in FIG. 17. Therefore, ifthe Fresnel reflecting mirror 1 is urged against the Fresnel reflectingmirror holder frame 12, the Fresnel reflecting mirror 1 can then becurved in the desired form.

[0099] The feature of the instant example is that whenever the appliedurging force has at least some strength, the given shape of curvature isobtainable. It is thus possible to curve the Fresnel reflecting mirror 1constantly in the given form independent of conditions such as ambienttemperature.

[0100] In the instant example, too, it is preferable to satisfycondition (2) or (2-1) in Example 3.

[0101] Example 5 of the present invention is now explained. In theinstant example, too, the Fresnel reflecting mirror is curved. In theinstant example, a Fresnel reflecting mirror support 14 is used. TheFresnel reflecting mirror support 14 is similar in structure to theFresnel reflecting mirror holder frame 12. In the instant example, too,the surface of the support in contact with a Fresnel reflecting mirror 1is curved on the basis of the pre-calculated amount of curvature of theFresnel reflecting mirror 1.

[0102] In this example, however, a number of suction holes 16 are formedin the surface 15 of the support as shown in FIG. 18. As suction forceis applied via the suction holes 16, the Fresnel reflecting mirror 1 iscurved following the shape of the surface 15. In this way, the Fresnelreflecting mirror 1 in the instant example can be curved. It isconsequently possible to make correction for decentration aberrationproduced at the Fresnel reflecting mirror 1.

[0103] It is noted that the substrate of the Fresnel reflecting mirror 1should preferably be thin. The thinner the substrate, the weaker theapplied suction force becomes, resulting in no need of any bulky suctiondevice.

[0104] More preferably, the following condition (2-2) should besatisfied.

t/ED<0.005  (2-2)

[0105] If the aforesaid condition (2-2) is satisfied, the Fresnelreflecting mirror 1 can then be more easily curved in conformity withthe surface 15, with weaker suction force.

[0106] Example 6 of the present invention is now explained. This exampleis directed to a mechanism for mounting a Fresnel reflecting mirror 1 ata predetermined position of a display system while it is kept in a givenattitude. Here the Fresnel reflecting mirror 1 is previously curved in agiven form. FIG. 19(a) is a perspective view of the instant example, andFIG. 19(b) is a top view of a mounting member.

[0107] As depicted in FIG. 19(b), the Fresnel reflecting mirror 1 isprovided in its one side with a cutout 1′ that serves as a positioningmeans. On the other hand, a mounting member shown generally at 17comprises a claw part 17 ₁, a pair of left and right claw parts 17 ₂ anda positioning projection 17 ₃. Combined with the claw parts 17 ₂, theclaw part 17 ₁ functions as a gripping means. The positioning projection17 ₃ is located between the pair of left and right claw parts 17 ₂. Theclaw part 17 ₁ cooperates with the claw parts 17 ₂ to support theFresnel reflecting mirror 1 while its one side is gripped between them,using their resilient force.

[0108] The Fresnel reflecting mirror 1 is forced from the one sidehaving cutout 1′ in between the claw part 17 ₁ and the claw parts 17 ₂,whereupon the cutout 1′ is fitted over the positioning projection 17 ₃of the mounting member 17. Consequently, the aforesaid one side of theFresnel reflecting mirror 1 is wedged between the claw part 17 ₁ and theclaw parts 17 ₂, where it is gripped and held.

[0109] Attachment or detachment of the Fresnel reflecting mirror 1 canthus be repetitively carried out. Even when the attachment or detachmentis repeated over and over, the Fresnel reflecting mirror 1 can be fixedconstantly at the same position.

[0110] Example 7 of the present invention is now explained. The instantexample is directed to an illumination means for a display device 3. Anoptical path diagram is shown in FIG. 20. The display device 3 used maybe either a transmission type two-dimensional display device or areflection type two-dimensional display device. In either case, a lightsource 5 is located at a position conjugate to an exit pupil E1 of arelay optical system.

[0111] With such an arrangement, light rays from the light source 5 arefocused on the exit pupil E1 of the relay optical system without a loss.On the other hand, the exit pupil E1 and final pupil E0 of the relayoptical system are conjugate to each other, and the eyeball of anobserver is located at the position of the final pupil E0. Therefore,the light rays from the light source 5 can arrive at the eyeball of theobserver without a loss. As a consequence, bright observed images can beobtained with reduced power. In FIG. 20, reference numeral 4 indicates acondenser lens for illumination purposes.

[0112] In FIG. 20, the display device 3 is of the transmission type. Itis noted, however, that when the display device used is of thereflection type, the light source 5 and condenser lens 4 must be locatedon the side of an eyepiece optical system 1.

[0113] In FIG. 20, the eyepiece optical system 1 and relay opticalsystem 2 are also shown as a transmitting lens. Even when a reflectingoptical system or any other desired optical element is relied upon,however, it is possible to take a similar layout as mentioned above.

[0114] Thus, the whole optical system according to the instant exampleis set up in such a way that the exit pupil E1 and final pupil E of therelay optical system 2 have conjugate relations to each other.

[0115] More preferably, some diffusion capability should be imparted tothe eyepiece optical system 1. This diffusion capability makes itpossible to increase the size of a pupil image (pupil diameter) at thefinal pupil E0. If the size of the pupil image at the final pupil E0 islarger than the size of the pupil of the observer, no limitation is thenimposed on the position of the eyeball (the iris) of the observer. Inother words, even with the eyeball of the observer deviating more orless from the final pupil E0, the light rays are incident on theeyeball; even with a slight displacement of the eyeball of the observer,images can be observed. It is thus possible to provide aneasy-to-observe display system.

[0116] Conversely, when the pupil image at the final pupil E is equal insize to the pupil of the eyeball of the observer, the size of the exitpupil E1 of the relay optical system 2 can be diminished. The reason isthat the eyepiece optical system 1 has diffusion capability; even whenthe exit pupil E1 of the relay optical system 2 is small, the diffusionaction ensures the same effect as is the case where the diameter of theexit pupil E1 of the relay optical system 2 is large. As a result, thereis a margin in the ability of the relay optical system 2 to correct foraberrations, which contributes to a resolving power improvement. Inaddition, it is possible to enlarge the display screen or achieve sizereductions of the display system.

[0117] Preferably, the eyepiece optical system 1 should satisfy thefollowing condition (3) with respect to its diffusion capability.

D<40°  (3)

[0118] Here D (° ) is the value of the full width half maximum on agraph indicative of the diffusion characteristics.

[0119] As already referred to herein, the diffusion characteristics arerepresented in terms of the given curve (graph) with diffusion angle asabscissa and light intensity as ordinate. In most cases, this curve isbilaterally almost symmetrical with respect to a given diffusion angle(e.g., 0°). There are then two angles where the maximum intensityreduces by half. In other words, the full width half maximum means thewidth between those two points. As a matter of course, the value isgiven by the diffusion angle indicated by that width. It is noted thatthe diffusion characteristics are not always required to have symmetry.

[0120] As the upper limit of 40° to the condition (3) is exceeded, thereis a drop of illumination efficiency upon the light source 5 projectedonto the final pupil E0. Consequently, a very bright light source isrequired for the light source 5, resulting in a failure to meet powersaving requirements.

[0121] More preferably, condition (3-1) should be satisfied.

D<20°  (3-1)

[0122] By satisfaction of this condition (3-1), further power savingsare achievable.

[0123] Most preferably, condition (3-2) should be satisfied.

D<10°  (3-2)

[0124] By satisfaction of this condition (3-2), the greatest possiblepower savings are achievable.

[0125] In the value range for the aforesaid condition, the diffusioncharacteristics are determined such that the {fraction (1/10)} fullwidth becomes at most three times the full width half maximum. Thismakes the illumination effect more efficient. The {fraction (1/10)} fullwidth used herein is understood to mean the width between two pointswhere {fraction (1/10)} of the greatest intensity is obtained. As amatter of course, the value is given by the diffusion angle indicated bythat width.

[0126] To be specific, the following conditions should preferably besatisfied in compliance with the aforesaid conditions (3), (3-1) and(3-2).

d<120°  (4)

d<60°  (4-1)

d<30°  (4-2)

[0127] Here d is the value of the {fraction (1/10)} full width on thegraph indicative of the diffusion characteristics.

[0128] By satisfaction of these conditions, bright images can beobserved even when the light source 5 used is of the very low outputtype.

[0129] More preferably, an LED should be used for the light source 5.This ensures efficient illumination. The LED light source has goodemission efficiency so that power consumptions can be kept low.

[0130] Alternatively, LEDs having wavelengths corresponding to R, G andB may be used as light sources. These LEDs, each having high colorpurity, can be so used for sequential illumination that the imagesdisplayed can be rendered in vivid colors.

[0131] Preferably, the following condition (5) should be satisfied.

WL<10 W  (5)

[0132] Here WL is the power consumption of the light source.

[0133] At a power consumption of 10 W or lower, images can be observedwith a battery or other power source over an extended period of time.

[0134] As schematically shown in FIG. 21, the display system of thepresent invention may also have surgical applications where surgicalmicroscopes, endoscopes, etc. are used. Of these tools, a surgicalmicroscope is of large size and includes many movable parts. For thisreason, it is necessary to apply a sterilization cover or the like overthe whole. In this case, especially if the light source used has largepower consumption, there is a problem that the heat of the light sourceis built up within the sterilization cover. This heat must be removed bymeans of an otherwise unnecessary separate means. Thus, it is of vitalimportance for a compact display system to make use of a light sourcehaving reduced power consumption.

[0135] More preferably, it is important to satisfy:

WL<1 W  (5-1)

[0136] By satisfaction of this condition, it is possible to achieve afurther reduction in the power consumption of a battery for driving thesystem. In other words, it is possible to reduce the size of thebattery, thereby achieving further size and weight reductions.

[0137] In the example shown in FIG. 21, a stand 18 is movable. A Fresnelreflecting mirror 1 is attached to an end 18′ of the stand 18 by meansof such a mounting member 17 as used typically in Example 5. Thisenables the “attachment” or “detachment” of the Fresnel reflectingmirror 1. Here again, the Fresnel reflecting mirror 1 is of the givencurved shape.

[0138] At a given position on the stand 18, a display unit 9 comprisinga display device 3, a relay optical system 2 and a light source (notshown) is mounted. Various images appearing on the display device 3 areprojected near the Fresnel reflecting mirror 1 via the relay opticalsystem 2, so that an operator can view the images via the Fresnelreflecting mirror 1. Images appearing on the display device 3, forinstance, include images from endoscopes, images from surgicalmicroscopes and TV images. The results of pre-operative inspections aswell as images such as CT images, 3D graphic images resulting from theCT images and MRI images, too, may be displayed on the display device.

[0139] It is noted that in such display systems for medical purposes,the Fresnel reflecting mirror 1 may possibly have been contaminatedduring operation. It is thus desired that after each use, the Fresnelreflecting mirror 1 be replaced by new one.

[0140] As shown in FIG. 22, the display system of the present inventionmay also be designed as a portable compact one. In FIG. 22, one displayunit 19 and one Fresnel reflecting mirror 1 are located on a substrate20 of the system body. The Fresnel reflecting mirror 1 is mounted insuch a way as to be foldable or erectable. The display unit 19 islocated at a position that, upon the Fresnel reflecting mirror 1 foldeddown, is in no contact with the Fresnel reflecting mirror 1.

[0141]FIG. 23 is a modification to FIG. 22. In FIG. 23, two displayunits 19L, 19R and one Fresnel reflecting mirror 1 are located on asubstrate 20 of the system body. The Fresnel reflecting mirror 1 ismounted in such a way as to be foldable or erectable. The display units19L, 19R are located at a position that, upon the Fresnel reflectingmirror 1 folded down, is in no contact with the Fresnel reflectingmirror 1.

[0142] The display units 19L and 19R are located at a given interval.The image of an exit pupil (final pupil) of a relay optical system builtin the display unit 19L is formed at a position E0L. On the other hand,the image of an exit pupil (final pupil) of a relay optical system builtin the display unit 19R is formed at a position E0R. Accordingly, imagescan be observed by both eyes while the left and right eyeballs of anobserver are in alignment with the positions of the final pupils E0L andE0R.

[0143] Suppose now that the display units 19L and 19R were located withthe respective optical axes intersecting at a given angle, and thatimages of parallax were displayed on the display devices built in thedisplay units 19L and 19R. Then, imagewise light of parallax is incidenton the left and right eyeballs of an observer, so that the observer canview a 3D image.

[0144] So long as the final images E0L and E0R are at a symmetricposition with respect to the Fresnel reflecting mirror 1, only therequirement for the curvature of the Fresnel reflecting mirror 1 is tocorrect for decentration aberration with respect to one pupil E0L orE0R.

[0145] Even with the system of FIG. 21 to which the same construction isapplied, it is possible for the observer to view 3D images.

What we claim is:
 1. An eyepiece optical system, comprising: a substratewith a Fresnel surface formed thereon, wherein the Fresnel surfacecomprises rotationally symmetric concentric zones, and the substrateincludes at least a curved area.
 2. The eyepiece optical systemaccording to claim 1, wherein the substrate comprises a flat part and acured part.
 3. The eyepiece optical system according to claim 1, whereinthe substrate is cylindrically curved in one direction of orthogonaldirections.
 4. The eyepiece optical system according to claim 3, whereinthe cylindrical curvature lies at a central portion of the Fresnelsurface that has an approximately flat peripheral portion.
 5. Theeyepiece optical system according to claim 3, wherein the one directionis orthogonal to a plane including a light ray incident on the Fresnelsurface and a light ray emerging from the Fresnel surface.
 6. Theeyepiece optical system according to claim 5, wherein the cylindricalcurvature lies only at the central portion of the Fresnel surface. 7.The eyepiece optical system according to claim 1, wherein the Fresnelsurface is a reflecting surface or a transmitting surface.
 8. Theeyepiece optical system according to claim 1, wherein the Fresnelsurface is a rotationally symmetric aspheric surface.
 9. The eyepieceaccording to claim 1, which has diffusion capability and satisfiescondition (3): D<40°  (3) where D (°) is a value of a full width halfmaximum on a graph indicative of diffusion characteristics.
 10. Aneyepiece, comprising: a substrate with a Fresnel surface formed thereon,wherein the Fresnel surface comprises rotationally symmetric concentriczones and the substrate is in a plane-parallel shape, and a holdermember for holding the substrate, wherein the holder member has a recessin which the substrate is held.
 11. The eyepiece optical systemaccording to claim 10, wherein: the holder member is formed at both endswith raised edges, a bottom surface of the holder member between theraised edges is a flat surface, each of opposite surfaces of the raisededges is in contact with each of both ends of the substrate, and aspacing between the opposite surfaces of the raised edges is smallerthan a spacing between both ends of the substrate.
 12. The eyepieceoptical system according to claim 10, which satisfies condition (2):t/ED<0.05  (2) where ED is a diagonal length of the Fresnel reflectingmirror 1, and t is a thickness of the Fresnel reflecting mirror
 1. 13.The eyepiece optical system according to claim 10, wherein: the holdermember is formed at both ends with raised edges, a bottom surfacebetween the raised edges is a curved surface, each of opposite surfacesof the raised edges is in contact with each of both ends of thesubstrate, and the substrate is curved in conformity with the curvedsurface to hold the substrate in place.
 14. The eyepiece optical systemaccording to claim 10, wherein: the holder member has a holding surfaceprovided with suction holes, and the substrate is sucked via the holdingsurface to hold the substrate in place.
 15. The eyepiece optical systemaccording to claim 10, comprising: a substrate with a Fresnel surfaceformed thereon, wherein the Fresnel surface comprises rotationallysymmetric concentric zones, and the substrate in a plane-parallel shapeand includes a cutout at one end, and a holder member for holding thesubstrate in place, comprising a first holder part, a second holder parthaving two holder elements, and a projecting part, wherein the firstholder part is located in opposition to the second holder part and theprojecting part, the projecting part is located between the two holderelements, and a width of the projecting part is substantially the sameas that of the cutout.
 16. A display system, comprising: a displaydevice having a display portion on which an image is to be displayed, arelay optical system having an entrance pupil and adapted for projectionof the image, and an eyepiece optical system as recited in claim 1 or10, wherein: the eyepiece optical system forms a final pupil that is animage of the entrance pupil at a given position, and the relay opticalsystem and the eyepiece optical system are located such that an axialchief ray emerging from the relay optical system is obliquely incidenton the eyepiece optical system, with the proviso that the axial chiefray is defined by a light ray emerging from a center of the displayportion, and passing through the relay optical system and then through acenter of an exit pupil of the relay optical system.
 17. The displaysystem according to claim 16, which satisfies condition (1):0<|E/EPD|<2  (1) where EPD is a diameter of the exit pupil of the relayoptical system, and E is an amount of aberration at a position of thefinal pupil.
 18. The display system according to claim 16, which furthercomprises a light source for illuminating the display device, whereinthe light source is a light-emitting diode.
 19. The display systemaccording to claim 16, which further comprises a light source forilluminating the display device, wherein the light source has a powerconsumption of 10 W or less.
 20. The display system according to claim15, which further comprises: a supporting column, a first arm joined tothe supporting column, and a second arm joined to the first arm,wherein: the display device and the relay optical system are held by thesupporting column or the first arm at a position higher than theeyepiece optical system, the eyepiece optical system is held by thesecond arm, and the display device displays an image obtained from asurgical microscope or endoscope.
 21. The display system according toclaim 15, which further comprises a holder member for holding thedisplay device, the relay optical system and the eyepiece opticalsystem, wherein: the display device and the relay optical system arelocated in opposition to the eyepiece optical system, the eyepieceoptical system is foldable with respect to the holder member, and thedisplay device and the relay optical system are located at a positionaway from the eyepiece optical system while the eyepiece optical systemis folded down.